We report here a new approach for determining frequency-dependent real and imaginary components of the refractive index of crystalline solids directly from the observation of the infrared spectra of the corresponding aerosols. The interplay between scattering and absorption observed in large (> 0.5-pm-radius) particles allows us to properly scale the imaginary component, determined from the absorption spectrum of small (around 0.3-pm-radius) aerosols, using calculations based on the Mie scattering theory. Once the imaginary indices are properly scaled, the corresponding real indices are determined through a Kramers-Kronig analysis. The method is applied to the study of water ice aerosols, and comparisons with previous measurements confirm that the method is sound and accurate, Reported here is a detailed study of the temperature dependence of the refractive index of ice. Experiments are reported over the temperature range from 130 to 210 K, which includes the region of interest for the study of polar stratospheric clouds (PSC's).

The refractive indices of nitric acid trihydrate (NAT) have been determined from the infrared spectra of laboratory generated aerosols. The aerosols are formed via homogeneous nucleation in a flow cell with separate regions for nucleation and observation, allowing for independent control of the temperature conditions in these regions. A spectrum of small, non‐scattering particles is recorded to determine the frequency dependent imaginary refractive index, within a scaling factor. A subtractive Kramers‐Kronig routine is then used to calculate the real index. The scaling factor for the imaginary indices is determined by fitting a spectrum associated with larger, scattering particles, which depends on both the real and imaginary portions of the refractive indices. The complex refractive indices of NAT are reported over the range 700 cm−1 to 4000 cm−1. While in good qualitative agreement with previously reported results, there are significant quantitative differences which are discussed.

Abstract. A low-temperature flow cell has been used in conjunction with a Fourier transform infrared (FT-IR) spectrometer to study sulfuric acid/water aerosols. The aerosols were generated with a wide range of composition (28 to 85 wt %), including those characteristic of stratospheric sulfate aerosols, and studied over the temperature range from 240 K to 160 K. The particles exhibited deep supercooling, by as much as 100 K below the freezing point in some cases. Freezing of water ice was observed in the more dilute (<40 wt % sulfuric acid) particles, in agreement with the predictions of Jensen et al. and recent observations by Bertram et al. In contrast with theoretical predictions, however, the entire particle often does not immediately freeze, at least on the timescale of the present experiments (seconds to minutes). Freezing of the entire particle is observed at lower temperatures, well below that characteristic of the polar stratosphere.

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